Improve agentless mode messaging for host-level metrics

Updated the messaging and UI to clarify unavailability of host-level metrics, such as CPU, RAM, and disk space, in agentless mode. Added clear formatting and new functions to handle missing metrics gracefully in the frontend.

backend/app/services/infra_probe.py

@@ -17,10 +17,10 @@ class DiskSpaceProvider:
 class NullDiskSpaceProvider(DiskSpaceProvider):
     async def get_free_bytes(self, target_host: str) -> DiskSpaceProbeResult:
         return DiskSpaceProbeResult(
-            source="none",
+            source="agentless",
             status="unavailable",
             free_bytes=None,
-            message=f"No infra probe configured for host {target_host}. Add SSH/Agent provider later.",
+            message=f"Agentless mode: host-level free disk is not available for {target_host}.",
         )